Vehicle transmissions with manually operated electrical remote control of gear shifting are well known, especially for off-road earth moving vehicles and the like. An electrical remote shift system offers several advantages such as eliminating complicated control linkages and enhancing vehicle design flexibility. There is a need, however, to provide automatic shifting in order to optimize the machine performance and to reduce the demands on the human operator. In the prior art, automatic transmission control systems have been proposed in which an electronic control system controls the shifting in response to selected operating parameters of the vehicle.
The Nelson U.S. Pat. No. 3,448,640 discloses an automatic transmission control system in which the shift points are a function of throttle position and transmission output speed. The system of this patent includes a plurality of speed range channels which receive input signals from a manual selector switch. A gate means or comparator in each channel compares a throttle position signal and a speed signal and when a predetermined relationship occurs the gate is enabled and an output is produced to call for the shift of the transmission ratio. The comparator is described as being enabled by a certain relation of the throttle signal and speed signal and when it is enabled the selector signal is gated through the comparator to an output terminal.
Schneider et al U.S. Pat. No. 3,805,604 discloses an automatic transmission control system in which a shift signal is produced by comparison of a vehicle speed signal with a plurality of speed reference signals. The vehicle speed signal is applied to one input of each of a plurality of "speed switches" which are operational amplifiers, each having adjustable set points corresponding to desired speed ranges. The set points are modified in accordance with throttle position. Further, the operational amplifiers are provided with switching hysteresis to separate the upshift and downshift points. The outputs of the operational amplifiers or speed switches are applied through a logic circuit which controls the solenoid valves for selecting the gear range.
The Ito et al U.S. Pat. No. 3,750,495 patent discloses a digital control system for automatic transmissions with provisions of means to alleviate shock during the shift from one gear to another. A logic circuit determines the shift points as a function of throttle position and speed. Means are provided for controlling the pressure to the planetary gear clutches during the gear changes. Wakamatsu et al U.S. Pat. No. 3,724,293, Ito et al U.S. Pat. No. 3,646,835, and Shimwell et al U.S. Pat. No. 3,122,940 are all directed to electronic transmission control systems in which the transmission shift points are controlled as a function of both vehicle speed and torque demand on the engine. They also include manual gear selector controls providing an input to the electronic system.